1. FIELD OF THE INVENTION
The present invention relates to information recording media, such as an optical disc or an optical card, on which information are recorded in accordance with optical features. More particularly, it relates to techniques for inhibiting the illegal duplications of the information recording media.
2. DESCRIPTION OF THE RELATED ART
A CD-ROM (compact disc read only memory), which is a kind of optical disc, can be mentioned as a typical example of information recording media on which information are recorded by affording optical features thereto.
Many types of CD-ROM adopt the CLV (constant linear velocity) system in which data are recorded using pits of regular intervals. This CLV system can make the data capacity of the CD-ROM larger than the CAV (constant angular velocity) system in which data are recorded using pits of intervals determined in accordance with the radial positions of the CD-ROM so that angles with respect to the center thereof may be held constant at all times. Besides, in the CD-ROM's, frames are set as record units irrespective of the CLV system and the CAV system. As illustrated in FIG. 18 of the accompanying drawings, each frame 1 is composed of data of fixed length, and a sync signal 2 affixed to the head of the data.
Incidentally, it is also possible that the CLV system and the CAV system coexist on one optical disc.
By way of example, according to a technique disclosed in the official gazette of Japanese Utility Model Registration Application Publication (KOKOKU) No. 44927/1993, an optical disc is provided with a record area based on the CLV system and a record area based on the CAV system, and still picture data, etc. are recorded in the latter record area. In addition, according to a technique disclosed in the official gazette of Japanese Patent Application Publication (KOKOKU) No. 33470/1993, an optical disc is provided with a record area based on the CLV system and a record area based on the CAV system, and the positions of the respective record areas are separately recorded as address information. The address information are utilized for the change-over between the rotational controls of a disc motor for the record area based on the CLV system and the record area based on the CAV system.
In this specification, an "original disc (original version)" is intended to mean a CD-ROM which is lawfully produced from a "legal master disc (legal master version)". On the other hand, a "copy disc (copy version)" is intended to mean a CD-ROM which is unlawfully produced from an "illegal master disc (illegal master version)" that is, in turn, prepared from the original disc.
At present, CD-ROM's are extensively utilized for recording digital data such as various sorts of software, document data and image data.
Such digital data which are recorded on the CD-ROM's are usually under the protection of copyrights. Nevertheless, the production and sale of illegal copy discs of the CD-ROM's have heretofore recurred.
Especially in the fields of software for games and electronic publications including independent volumes, dictionaries etc., the production of the illegal copy discs has often been organically made and has formed a factor for hampering normal commercial transactions.
The above mentioned copy discs of the optical disc such as CD-ROM can be produced from the illegal master disc thereof. This illegal master disc of the copy discs is prepared from a mold in which the shapes of pit arrays recorded in the information recording surface of the original disc of the optical disc are transferred by the use of a plastics material or the like. The molding technique, however, often fails to exactly produce the copy discs for the reason that the shapes of the pit arrays need to be transferred after the protective film of the original disc is removed to expose the pit array surface, so the arrayed pits are deformed at the stage of removing the protective film. Especially in a case where the protective film is rigid, the production of the copy discs of the optical disc is very difficult.
Therefore, the illegal master disc of the copy discs of the optical disc is ordinarily prepared using data read out of the original disc of the optical disc, just as in the case of producing the original disc from the legal master disc of the optical disc.
Meanwhile, in order to prevent a pirate from producing such copy discs of the optical disc, a copyright has heretofore been indicated on the front surface of the original disc of the optical disc by carving or printing characters, a graphic or the like.
In addition, examples to be mentioned below have been known as prior-art techniques for discriminating the illegal copy disc of the optical disc from the legal original disc thereof.
The technique disclosed in the official gazette of Japanese Patent Application Laid-open No. 286768/1992 utilizes the fact that the sync signal 2 stated before with reference to FIG. 18 cannot usually be read out as data. With this technique, another signal is substituted for part of the sync signal 2 and is recorded beforehand. In playing back an optical disc, the presence or absence of the substitute signal is especially detected, and the optical disc is decided as an illegal copy disc in the absence of the substitute signal.
In the official gazette of Japanese Utility Model Registration Application Laid-open No. 20884/1990, it is disclosed to form a visible pattern on the front surface of the signal recording layer of an optical disc. Also, it is disclosed in the official gazette of Japanese Patent Application Laid-open No. 179941/1990 that a pattern or the like which is visible is formed by changing the pit shapes of the signal surface of an optical disc.
Regarding the technique disclosed in the official gazette of Japanese Patent Application Laid-open No. 286768/1992, the sync signal 2 including the substitute signal can be, in itself, reproduced. Therefore, it is not very difficult to produce an illegal copy disc of the optical disc in which the sync signal 2 including the substitute signal is stored.
On the other hand, according to the technique disclosed in the official gazette of Japanese Utility Model Registration Application Laid-open No. 20884/1990 or Japanese Patent Application Laid-open No. 179941/1990, the step of forming characters, a graphic or the like needs to be separately performed in addition to a conventional process for preparing the master disc of the optical disc.